1. Field of the Invention
The present invention relates to a photovoltaic device which generates electromotive force when radiated with light.
2. Description of the Prior Art
Desirably, the light-receiving electrode layer (hereinafter referred to as transparent electrode layer) of a photovoltaic device which generates electromotive force on exposure to light should be transparent in order that light can be transmitted to the semiconductive layer which contains the photo-active layer providing photoelectric conversion. Accordingly, conventional transparent electrode layers are mainly composed of a transparent conductive oxide (TCO) such as In.sub.2 O.sub.3, SnO.sub.2, or ITO which are, respectively, the oxidized products of indium (In), tin (Sn), and a combination thereof. It is rarely composed of a thin metal layer. Such an electrode composed of TCO has a sheet resistance value of about 10 through 50 .OMEGA./.quadrature., which is 3 orders of magnitude higher than that of a thin metal layer made from aluminum having identical thickness. This causes those electrode layers made from TCO to cause a slight loss of power (due to resistance) and to lower the current collecting efficiency.
To prevent lowering of &he current collecting efficiency, Japanese Patent Application Laid-Open No. 59-50576 (1984) had proposed providing the light-receiving-side with gridshaped current-collecting electrodes made from a metallic substance.
Because of its lower resistance value that of a transparent oxide electrode layer, a current collecting metal electrode layer can prevent degradation of current collecting efficiency. On the other hand, since the current-collecting metal electrode blocks light which should reach the photoactive layer, the metal electrode unavoidably reduces the effective light-receiving area where photoelectric conversion is executed.
To solve this problem, improved photovoltaic devices were proposed by Japanese Patent Application Laid-Open Nos. 60-0149178 (1985), 61-20371 (1986), and by Japanese Utility Model Application Laid-Open No. 61-86955 (1986), respectively. These photovoltaic devices respectively reduce resistance loss of the transparent electrode layer without significantly sacrificing effective light-receiving area despite the presence of highly-resistant TCO or thin metal layer used for composing the transparent electrode layer. FIG. 1 is the sectional view of the photovoltaic device proposed by one of the above references. Each photoelectric converter element SC.sub.1, SC.sub.2, SC.sub.3, . . . is respectively connected to an adjacent converter element by superimposing the transparent electrode layer 11, semiconductive layer 12, the first back electrode layer 13 composed of ohmic metal, insulative layer 14, and the second back electrode layer 15 having a resistance value lower than that of the transparent electrode layer 11, on transparent light-incident substrate 17. In this photovoltaic device, a plurality of connection conductors 18 made from the same material as the second back electrode layer 15 are inserted in contact holes 16 having their inner surfaces being surrounded by insulative layer 14, at a plurality of locations in the light-receiving region so that the transparent electrode layer 11 can be electrically connected to the second back electrode layer 15. A plurality of photoelectric converter elements SC.sub.1, SC.sub.2, SC.sub.3, . . . are formed on the transparent insulative substrate 17, where adjoining photoelectric converter elements are electrically connected to each other in series by combining the first back electrode layer 13 of one of the adjacent photoelectric converter elements with the second back electrode layer 15 of the other.
The photoelectric converter elements of the above-cited photovoltaic device electrically connect the high-resistance transparent electrode layer 11 and the low-resistance second back electrode layers 15 at a plurality of locations, whereby the current path in the transparent electrode layer 11 extends up to the connection with the low-resistance second back electrode layer 15, whereby resistance losses due to the transparent electrode layer 11 can be reduced without substantially decreasing the effective light-receiving area.
Because of low cost and easy fabrication into optional shape, glass is conventionally used for the insulative substrate 17. Nevertheless, since glass softens at about 550.degree. C., it cannot withstand heat treatment of more than 600.degree. C. As a result, when forming semiconductive layer 12 having semiconducting junctions, only processes such as evaporation, sputtering, plasma CVD, and photo CVD using low temperatures are applicable. This in turn confines the range of materials usable for semiconductive layer 12, and thus material can be selected only from amorphous silicon, amorphous silicon carbide, amorphous silicon germanium, amorphous germanium, and microcrystalline silicon, which are generated by means of plasma CVD or photo CVD and the like using a maximum substrate temperature of 300.degree. C.
FIG. 2 is an enlarged sectional view of the unction of the transparent electrode layer 11 and the second back electrode layer 15. Since the inner surface of contact hole 16 is covered with insulative layer 14, the transparent electrode layer 11 is not directly connected to the first back electrode layer 13. However, if the thickness of semiconductive layer 12 is less than 1 82 m, in particular, if the thickness is about 0.5 .mu.m, leakage current will be generated through the edge portion 12a of the junction, and accidental short circuits may occur.
The intervals between adjacent photoelectric converter elements and junctions (contact holes 16) constitute specific regions which do not contribute to the generation of power, thereby adversely affecting output power. These intervals are essential for connecting a plurality of photoelectric converter elements in series. Because of the precision needed for processing, the minimum size of the intervals is predetermined. Consequently, in order to reduce the regions which do not contribute to the generation of power, the size of contact holes 16 and the intervals between each contact hole 16 should be set to optimal values.
Etching processes using photomasks are conventionally used for removing layers for providing contact holes 16. However, it is quite difficult for etching processes to precisely locate finely spaced positions. Furthermore, etching requires a large number of steps to be executed, and cannot easily form fine contact holes 16.